One type of analog-to-digital converter, commonly referred to as a parallel or "flash" analog-to-digital converter, employs a group of voltage comparators, each of which has an analog signal input and a voltage reference input. The analog signal inputs of the comparators are electrically connected together to provide a single-ended input configuration. The reference voltage inputs receive reference voltages from a series-connected resistor network. The comparators provide output signals which form a "thermometer" code corresponding to the amplitude of the input signal. The thermometer code is then encoded into a digital signal having a binary format.
An analog-to-digital converter is fabricated typically as an integrated circuit. A flash analog-to-digital converter of the integrated circuit type that employs a single-ended input configuration uses the substrate as the primary return path for the input signal currents. The application of the same analog input signal to all of the comparators causes capacitive injection of substantial noise currents into the substrate. These noise currents can cause unacceptable levels of cross talk among the different circuits comprising portions of and the signals flowing within the integrated circuit. The presence of noise currents reduces the precision of the analog-to-digital converter. Significant amounts of return signal currents also flow through the reference voltage and power supply circuitry. Consequently, to ensure proper operation of the circuit, it is necessary that each of the return current paths have a low impedance connection to the analog input signal source ground. Such low impedance return connections are, however, difficult to achieve.